Loaded pontoon cavitation

[Rabbit929]

So it works fine unloaded, but I’ve noticed when loaded down pretty much max, I have a lot of cavitation issues intermittently with the prop.
It seems I can play with the trim a bit and make it mostly go away, (deeper DOES NOT Seem to be better.)

it’s not a huge deal but just curious what others have done for similarly loaded down pontoons that seem to start ventilating?

 

[Texasmark]

You are probably ventilating...sucking air into your slip stream. Usual causes are engine mounted too high, too much pitch for the load to be pushed, or obstructions preceding the new water that flows into the prop causing bubbles.

Unless you are unreasonably high, I'd guess that you have something disturbing the water ahead of the lower unit since trim has little effect. If it were a height problem tucking the trim all the way in should eliminate any ventilation. Propellers with "cupped" blade trailing edges are designed to reduce ventilation but the normal use is for Bass Boats (to name an application) with high trim angles and high speeds, and tight turns...on any boat, especially with boats with protruding keels..even if only 1" high.....(aluminum boats in particular...covering where the sheets on the bottom meet).

On too much pitch, the engine is turning the prop shaft, but the high pitch and heavy load mean that there is too much water to be pushed per revolution and the path of least resistance is the air above. Besides that too much pitch isn't good for your engine, especially if it's a 2 stroker.

These are some of the things I picked up over the years. May not be applicable in all cases but are places to look.

 

[Rabbit929]

I appreciate the reply, and yes it does seem like changing the trim helps even if im all the way down.
i should add its a 21ft sylvan dual log, and its almost as if with all the extra weight, the tubes are disturbing/displacing water infront of the prop, and im just grabbing a steady stream of bubbles.
idk if it will cause problems or not, i just would like to make sure i dont hurt anything.

 

[tpenfield]

If you have got the prop all the way down and not experiencing any wave action, then it may be cavitation, which is the water turning to a gas/vapor on the back of the blades.

Do check the engine height to make sure it is correct. But if all is right, then I would suggest going down a notch in propeller pitch.

 

[Texasmark]

I appreciate the reply, and yes it does seem like changing the trim helps even if im all the way down.
i should add its a 21ft sylvan dual log, and its almost as if with all the extra weight, the tubes are disturbing/displacing water infront of the prop, and im just grabbing a steady stream of bubbles.
idk if it will cause problems or not, i just would like to make sure i dont hurt anything.

Hurting anything????? Not staying within your recommended RPM range could lead to engine damage, especially if its frequently occurring event.

 

[roscoe]

"when heavily loaded"

If most of the weight is forward on the deck, can be lowering the front and raising the rear compared to lightly loaded operation.

Try having the people all shift back to midship or further, and see if problem goes away.

 

[Texasmark]

"when heavily loaded"

If most of the weight is forward on the deck, can be lowering the front and raising the rear compared to lightly loaded operation.

Try having the people all shift back to midship or further, and see if problem goes away.

That's a good point. Not that the toon is overloaded causing the condition, it's how it's loaded.....Thinking about it, the lounges are midship and forward.....so the passengers do what you said when using the lounge seats.

Edit: You know it's amazing the difference in sitting in front of your computer trying to analyze a problem with facts given, and standing by or sitting and riding in the problem equipment. Some things you just don't think about.

 

[Rabbit929]

Hurting anything????? Not staying within your recommended RPM range could lead to engine damage, especially if its frequently occurring event.

RPM rang is staying above 5500 per spec, but i just notice some erattic sounding ventilation noises, and shifting everyone towards the backisnt really an option, as the pontoons pretty full at that point. its only an issue at WOT, when trying to scoot to another part of the lake.
Like i said i dont think its the end of the world, but im wondering if installing a jack plate to lower the motor in the water further would help maybe, or maybe thats just how she sounds when shes pulling hard and only going 18mph. alone i got up to 29 on glasswater.

 

[Texasmark]

Hydraulic jack plate would be nice. You could set the height as a function of load as you go....butttt $$$. Manual jack plate would be fixed and for the engine to be set right for full loaded conditions, your top end would/should drop a blt due to the extra drag. If above 5500 on most engines, you have some room for a cupped prop, which grabs the water better it loads the engine more, but can/(is advertised to possibly) act as an additional inch of pitch...at high rpms...and give you a bit more top end.. If you can find the same mfgr. and pretty much the same type of prop as your current one with the only change being cupping of the trailing edge of the blade, that would cost you a bit over a hundred bucks and you would have a spare prop...your current one and drop your RPMs slightly. Changing mfgrs. or models can add unknown variables to your results.